Even if, for example, consciousness and matter in motion were distinct and distinguishable, this would be no argument against a science of physiological psychology. Cerebral and psychical phenomena form one series, and if we have at present no adequate science which concerns itself with this series, it is owing to ignorance of facts, not at all to logical limitations. Matter, time, space and the differential calculus may be as disparate as possible, but are brought together in the science of physics. If the psychologist can not be shut out of the physical world, still less can he be excluded from the sphere of the so-called normative sciences. If any one takes a modern work on ethics or esthetics and tries to separate the treatment of 'what is' from that of 'what ought to be' he will find himself engaged in an idle task.
It appears that the limits of a science are set largely by a psychological constant. A single science has practically the range that can be covered by a single mind or man. From Aristotle to Hobbes and Descartes there were philosophers who could master nearly the whole range of knowledge and advance it in whatever direction they cared to turn. But even in this period as knowledge accumulated, specialization began, and we find astronomers, anatomists and other students of particular sciences. After Galileo and Newton the physico-mathematical sciences became completely divorced from the descriptive natural sciences, while psychology remained under the shelter of philosophy. It was only in the second half of the nineteenth century that the accumulation of certain facts and theories warranted their becoming the chief interest of a psychologist, and even yet it is more usual for a man to pass through a psychological period than to be a permanent psychologist.
While the first result of increased knowledge has been the establishment of a number of sciences—say a dozen or a score—which have secured proselytes and to a certain extent limited and directed their activities, the further increase of knowledge must break down the artificial limitations. The late emergence of psychology has made easy an elective selection of material. We not only have psychologists who are also philosophers, but psychologists who are also physiologists, anatomists, pathologists, zoologists, anthropologists, philologists, sociologists, physicists or mathematicians. Psychology is and will increasingly become united with professions and arts, with education, medicine, music, painting and the rest. Even sciences remote from psychology, astronomy, for example, may have sufficient points of contact to occupy the entire time of a specialist. We not only have combinations between the orthodox sciences, but cross-sections through them, which may to advantage occupy the student, and which have full rights to be ranked as sciences. The phenomena of vision, for example, are scattered among the sciences of psychology, physics, physiology, anatomy, anthro-
